1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices. More particularly, the present invention relates to a loadlock chamber of semiconductor manufacturing apparatus and to a method of controlling the pressure in the chamber in the course of transferring wafers to and from the chamber.
2. Description of the Related Art
In general, a semiconductor device is fabricated by subjecting a wafer to such selective and repetitive processes as photolithography, etching, diffusion, chemical vapor deposition (CVD), ion implantation and metal evaporation processes. To this end, the wafer is mounted on a cassette, and the cassette is transferred to respective fabricating equipment for processing the wafers. Also, the wafers are transferred by a robot into position in the fabricating equipment.
A loadlock chamber is typically used in such a transfer of wafers to allow the processing conditions, e.g., pressure, within the fabricating equipment to be maintained while the wafers are moved in and out of the fabricating equipment. That is, the pressure within the loadlock chamber is regulated to provide a buffer between the atmospheric pressure prevailing in the production line outside the fabricating equipment and the vacuum pressure atmosphere within the fabricating equipment. To this end, the interior of the loadlock chamber is purged or filled with gas during the course of transferring the wafers to and from the fabricating equipment. A conventional technique of purging/filling the loadlock chamber will be described with reference to the FIGS. 1 and 2.
First, as shown in FIG. 1, a general loadlock chamber 10 includes a first door 12a adjacent a production line and a second door 12b adjacent a process chamber 14 or a transfer chamber 16. The first door 12a can be selectively opened and closed so that the cassette C having numerous wafers therein can be introduced into and withdrawn from the processing equipment. The second door 12b can be selectively opened and closed so that the wafers W can be introduced into or withdrawn from the process chamber 14 or transfer chamber 16. A vacuum pressure atmosphere is maintained with the process chamber 14 or transfer chamber 16.
The loadlock chamber 10 is connected to a vacuum pump 18 and an exhaust line 20 at one side thereof. The vacuum pump 18 is operative to produce a vacuum within the loadlock chamber 10. On the other hand, a gas supply 22 for supplying a purge gas to the interior of the loadlock chamber 10 is connected through a gas supply line 24 to another portion of the loadlock chamber 10. Control valves 26a, 26b for controlling a flow of fluid in response to a signal generated by a controller (not shown in the drawings) are respectively installed in the exhaust line 20 and the gas supply line 24.
The vacuum/purge operations of the loadlock chamber 10 will be described now with reference to FIG. 2.
The cassette C having numerous wafers W mounted therein is transferred from the production line into the loadlock chamber 10 while the first door 12a is open (ST100). Then the controller closes the first door 12a to thus seal off the atmosphere within the loadlock chamber 10 (ST102).
Subsequently, while the gas supply line 24 is cut off by the control valve 26b, the controller controls the vacuum pump 18 and the control valve 26a such that air is withdrawn from the loadlock chamber 10 through exhaust line 20. Thus, a vacuum (negative pressure) is produced within the loadlock chamber 10 (ST104). At the same time, the pressure within the loadlock chamber 10 is measured by a sensing unit (not shown in the drawings). The controller uses the sensed pressure to determine when the pressure has reached a predetermined level, namely, a level corresponding to that within the process chamber 14 or transfer chamber 16 (ST106).
Once the pressure in the loadlock chamber 10 has reached the predetermined level, the controller opens the second door 12b (ST108), and controls a robot R provided within the process chamber 14 or the transfer chamber 16 to thus take out the wafers W and subsequently transfer the wafers W to a position required for their being processed (ST 110).
After a wafer is processed, the controller transfers the processed wafer W back into the cassette C in the loadlock chamber 10 (ST112). The controller closes the second door 12b once all of the respective processed wafers W have been mounted in the cassette C (ST114).
Next, the control valve 26b is opened so that purge gas is supplied into the loadlock chamber 10 through the gas supply line 24 while the control valve 26a cuts off the exhaust line 20 (ST116). Furthermore, the sensing unit senses the increase in pressure in the chamber 10. Accordingly, a constant pressure equal to that prevailing in the production line is produced in the loadlock chamber 10 (ST118).
Once the pressure within the loadlock chamber 10 equals the atmospheric pressure atmosphere of the production line, the controller opens the first door 12a, and the cassette C is transferred from the loadlock chamber 10 (ST120).
As mentioned above, the valves 26a, 26b are opened and closed when changing the pressure within the loadlock chamber 10 from a vacuum pressure level to an atmospheric pressure level, or vice versa. In particular, the control valve 26b installed in the gas supply line 24 is closed and then control valve 26a installed in the exhaust line 20 is opened, when the pressure within the interior of the loadlock chamber 10 is to be reduced to the level of a vacuum. Conversely, the control valve 26a in the exhaust line 20 is closed, and the control valve 26b in the gas supply line 24 is opened when the purge gas is to be introduced into the loadlock chamber 10 to bring the pressure therein up to atmospheric pressure.
However, with respect to the former operation, the temperature of the loadlock chamber 10 is momentarily reduced to about −40° C. when the loadlock chamber 10 is suddenly exposed to the vacuum pressure and the supply of the purge gas is cut off. At this time, air and various gases remaining in the loadlock chamber 10 are solidified by the momentary temperature drop, thereby forming particles. These particles are blown around the loadlock chamber by an eddy phenomenon created by the rapid change in pressure, and thus pollute or damage the wafer(s) W and the interior of the loadlock chamber 10.
A system has been developed in an attempt to prevent such a rapid decrease in pressure within the loadlock chamber 10. This system is installed on the exhaust line and is operative to slowly reduce the pressure for a predetermined period of time at the initial stage in the operation of creating a predetermined level of vacuum pressure in the loadlock chamber. The system then gradually increases the rate at which the pressure is reduced until the predetermined level of vacuum pressure is attained.
However, such a system merely delays the time required to produce the desire level of vacuum pressure within the loadlock chamber. That is, the pressure is rapidly decreased after the lapse of a predetermined period of time, whereby an eddy phenomenon occurs in the loadlock chamber 10. Therefore, this system is still subject to the problems of the polluting and damaging of the wafers W and the loadlock chamber 10.
Meanwhile, these problems also occur in the latter case mentioned above, i.e., the changing of the pressure within the loadlock chamber 10 from a vacuum pressure level to an atmospheric pressure level. That is, various gases and particles are produced, and are entrained by an eddy created when the purge gas is suddenly supplied while the source of the vacuum pressure is cut off. Thus, the wafers W and the loadlock chamber 10 are polluted and damaged as in the former case.